By default topics will be listed in the navigation bar Table of Contents in the order they were created. You can change the order by right clicking on the Table of contents icon in the navigation bar in the project structure view and selecting Settings. 

You can then drag and drop the topics to reorder them:

Click Update settings to save your changes.

Troubleshooting: I have renamed the topics, but the Table of Contents are not updating

If this issue occurs you can prompt the Table of Contents to update by taking the following steps;

1. un-tick one of the options in the Table of Contents topics list

2. tick the option again

3. select 'Update settings'